Rocks decompose slowly into soil, releasing minerals gradually over a period of years. Organic gardeners use many different minerals to increase the fertility of their soils, but it’s a long-term proposition. Some take months or years to fully break down into nutrient forms that plants can use, so one application may last a long time.
Rock on with these fertilizers:

• Chilean nitrate of soda: Mined in the deserts of Chile, this highly soluble, fast-acting granular fertilizer contains 16 percent nitrogen. It’s also high in sodium, however, so don’t use it on arid soils where salt buildup is likely or on salt-sensitive plants.

• Epsom salt: Containing magnesium (10 percent) and sulfur (13 percent), Epsom salt is a fast-acting fertilizer that you can apply in a granular form or dissolve in water and spray on leaves as a foliar fertilizer. Tomatoes, peppers, and roses love this stuff! Mix 1 tablespoon of Epsom salt in a gallon of water and spray it on when plants start to bloom.

• Greensand: Mined in New Jersey from 70 million-year-old marine deposits, greensand contains 3 percent potassium and many micronutrients. It’s sold in a powdered form but breaks down slowly.

• Gypsum: This powdered mineral contains calcium (20 percent) and sulfur (15 percent). Use it to add calcium to soils without raising soil pH.

• Hard-rock phosphate: This mineral powder contains 20 percent phosphorous and 48 percent calcium, which can raise soil pH — avoid it if your soil is already alkaline. It breaks down slowly, so use it to build the long-term supply of phosphorous in your soils.

• Soft-rock phosphate: Often called colloidal phosphate, soft-rock phosphate contains less phosphorus (16 percent) and calcium (19 percent) than hard-rock phosphate, but the nutrients are in chemical forms that plants can use more easily. This powder breaks down slowly, so one application may last for years in the soil. It also contains many micronutrients.

• Limestone: This mined product has various nutrient levels, depending on its source. It’s used primarily to raise pH, but dolomitic limestone, is high in calcium (46 percent) and magnesium (38 percent). This powder also comes in an easier to spread granular form. Calcitic limestone is high in calcium carbonate (usually above 90 percent). Conduct a soil test for pH and for magnesium to find out which kind of lime and how much to add to your soil.

• Rock dusts: Mined from various rocks, these powders often contain few major nutrients but are loaded with micronutrients and other essential elements needed in small quantities by plants. Use them as supplements to regular soil building and fertility programs.

• Sul-Po-Mag: Also known as K-Mag, this mineral source provides plants with readily accessible potassium (22 percent), magnesium (11 percent), and sulfur (22 percent). Use it to add magnesium to high pH soils without raising the pH.

• Sulfur: This yellow powder contains 90 percent elemental sulfur, which is used not only as a nutrient for plant growth, but also to lower the pH. Go easy on the application, however, because overuse can decrease soil microbe activity.